As a method for measuring (qualitatively or quantitatively) an objective component, an enzyme method using a redox reaction has been used widely. This method is, for example, a method in which an oxidizing substance is caused to be produced from an objective component to be measured, this oxidizing substance is caused to react with a color former that produces a color-developing compound by oxidation with the aid of an oxidizing enzyme, and the absorbance of developed color is measured. In this method, the degree of absorbance corresponds to the amount of the produced color-developing compound, the amount of the produced color-developing compound corresponds to the amount of the produced oxidizing substance, and the amount of the produced oxidizing substance corresponds to the amount of the objective component. That is, by detecting the developed color (the produced color-developing compound), the objective component can be measured indirectly through such a redox reaction.
As such a color former that develops color by oxidization, 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine, which releases methylene blue as a chromophore, has been known. Methylene blue is less liable to fade in color by re-reduction, whereby this color former allows highly sensitive detection to be conducted. Thus, it is expected that the color former will be used in various analyses.
When using this color former in, for example, a redox reaction as mentioned above, the color former is stored until use as a liquid reagent (wet type) obtained by dissolving the color former in a solvent such as water, a buffer, or the like on account of ease of handling, for example. Alternatively, the color former is stored until use as a test piece (dry type) obtained by fixing the color former on a base material such as a porous body and the like so that the redox reaction can be started by simply adding a sample. However, this color former is very unstable, and there is a problem that color is developed spontaneously before starting an artificial redox reaction by adding an enzyme as described above, i.e., during storage. The cause thereof is presumed to be that, for example, in the case of a wet type, the color former becomes unstable in the solvent, and in the case of a dry type, the color former becomes unstable by absorbing moisture from the air, and the like. Furthermore, regardless of whether a wet type or a dry type, it has been seen as a problem that the color former becomes unstable to cause color to be developed spontaneously also in the case where the color former is stored with exposure to light such as ultraviolet and the like.
When spontaneous color development of the color former occurs as described above, release of methylene blue may cause an increase in background absorbance in the measurement of an absorbance at the time of actual use, resulting in degraded measurement accuracy. In particular, in the color former, the released methylene blue has a property of being less liable to fade in color as described above. Thus, while there is an advantage of high sensitivity, a chromophore (methylene blue) released by natural oxidization also is less liable to fade in color as compared with other chromophores, thus posing a problem in that the influence thereof is difficult to be avoided. When the color former is stored in the liquid state, spontaneous color development that exerts an influence on the measurement is seen within about one day, and it would be difficult to use the color former as a reagent for an analysis within about three days, for example (Patent document 1).
In order to prevent such an influence of spontaneous color development, it is required to prepare a reagent of the color former every time a measurement is performed. This, however, leads to a complicated operation and also to a cost increase.    Patent document 1: JP 4(1992)-27839 A